Fluid injector

ABSTRACT

A fluid injector, particularly suitable for discharging liquid fuel to a boiler, has a tip valve for controlling fluid discharge through a passage, first and second ports for connection to an external fluid delivery and return circuit, a first fluid path in the injector from the first port to the vicinity of the discharge passage and a second fluid path in the injector from the vicinity of the discharge passage to the second port and a spill valve in the first fluid path; by delivering fluid to the first port the tip valve is closed and the spill valve is opened to cause fluid to be circulated through the injector and by delivering fluid to the second port only the tip valve is opened and the spill valve opens to promote discharge through the passage, and the spill valve is controllable to adjust the quantity of fluid returned to the return circuit thereby controlling the proportion of supplied fluid which is discharged through the passage.

United States Patent [191 [1111 3,8 9

Needham 1 Oct. 29, 1974 FLUID INJECTOR Primary Examiner-M. Henson Wood, Jr.

Assistant Examiner-John J. Love [75] Inventor' g Mlchael Needham Nausea Attorney, Agent, or FirmWaters, Roditi, Schwartz &

gland Nissen [73] Assignee: Tectron Engineering Limited,

I Bristol, England 57] ABSTRACT [22] Wed: 1973 A fluid injector, particularly suitable for discharging [21] Appl. No: 419,721 liquid fuel to a boiler, has a tip valve for controlling fluid discharge through a passage, first and second ports for connection to an external fluid delivery and [30] Foreign Apphcaumi pfmmy Data return circuit, a first fluid path in the injector from the NOV. 30, Great Britain first port to the of the discharge passage and a I second fluid path in the injector from the vicinity of [52] US. Cl 239/125, 239/126, 239/46 h discharge passage to the second port and a spill [51] Int. Cl BOSb 1/34 valve in the fi t fl id path; by delivering fl id to the [58] F'eld Search 239/127 464 first port the tip valve is closed and the spill valve is opened to cause fluid to be circulated through the in- [56] References C'ted jector and by delivering fluid to the second port only UNITED STATES PATENTS the tip valve is opened and the spill valve opens to. 2,743,137 4/1956 Wilson 239/125 Promote discharge through the Passage, and the Spill 3,395,863 8/1968 Johnson 239/125 valve is controllable to adjust the quantity of fluid re- 3,587,970 6/1971 Tindall 239/126 turned to the return circuit thereby controlling the 3,669,354 6/1972 Helyer 239/464 proportion of supplied fluid which is discharged through the passage.

13 Claims, 4 Drawing Figures 11, M rM/JLIM 0 3 22 8 s i V PAIENTEUum 2919M SHEEF 10$ 2 FLUID INJECTOR This invention relates to fluid injectors, particularly injectors for injecting liquid fuel into a furnace.

Injectors are known which comprise a fluid discharge passage having a tip sealing valve for controlling discharge of the fluid through the passage. It is known with such injectors to provide a spill passage for returning a variable amount of excess fluid to a fluid reservoir. Further it is frequently necessary or desirable with such injectors to circulate fluid through the body of the injector to a point adjacent to the discharge passage while the passage is sealed by the tip sealing valve in order to provide cooling to the injector when the injector has been shut down to protect it from heat radiated on to the injector from the burner or furnace with which the injector is associated.

According to the present invention, a fluid injector comprises a fluid discharge passage, a tip sealing valve for controlling fluid discharge through the passage, first and second ports for connection to an external fluid delivery and return circuit, a first fluid path in the injector from said first port to the vicinity of the discharge passage, a second fluid path in the injector from the vicinity of the discharge passage to the second port and a spill valve in said first fluid path, the spill valve and the tip sealing valve being arranged such that with fluid delivered to the first port the spill valve is subjected to a fluid differential pressure to open the spill valve to permit the fluid to be delivered to the vicinity of the discharge passage and the tip valve is subjected to a fluid differential pressure to close the discharge passage whereby the fluid returns via the second path to the second port; and with fluid delivered to the second port the tip valve is subjected to a fluid differential pressure to open the discharge passage and the spill valve is subjected to a fluid differential pressure tending to close the first fluid path to thereby promote discharge through the passage and means for adjusting the spill valve to permit a proportion of the fluid supplied to the second port to return along the first path thereby controlling the proportion of the supplied fluid which is discharged through the discharge passage.

The fluid injector is supplied with fluid under pressure at one or other of its two ports from an external fluid delivery and return circuit. A four port fluid changeover valve external of the fluid injector determines to which of the ports on the injector the fluid is delivered. When the fluid is delivered to the first port it subjects the spill valve to a fluid differential pressure which serves to open the valve to permit the fluid to be directed into the vicinity of the tip sealing valve. The fluid subjects the tip sealing valve to a differential fluid pressure which serves to cause the sealing valve to close off the fluid discharge passage and the fluid is returned to the second port and thence to the return circurt.

However when the changeover valve is operated to cause the fluid to be delivered to the second port on the injector the fluid is delivered to the vicinity of the discharge passage and the tip valve is subjected to a fluid differential pressure in the direction to open the discharge passage. The spill valve is subjected to a further fluid differential pressure which tends to close the spill valve thus preventing any fluid from being returned to the return circuit and ensuring that all the fluid is discharged from the injector through the fluid discharge passage.

It may be desired however that only a certain proportion of the supplied fluid is discharged through the discharge passage and that the remaining portion of the fluid is returned to the return circuit. To this end the spill valve is provided with adjustable means which enables the spill valve to be controlled thereby preventing it from being closed completely and allowing a con trolled proportion of the supplied fluid to be returned to the return circuit.

In order that the invention may be more readily understood it will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a sectional side elevation of a fluid injector when the injector is in a shut off condition,

FIG. 2 is a sectional side elevation of a changeover valve which may be used in conjunction with the fluid injector shown in FIG. 1;

FIG. 3 is a sectional side elevation of part of a fluid injector according to a further embodiment of the invention, and

FIG. 4 is a sectional side elevation of part of a fluid injector according to a still further embodiment of the invention.

Referring now to FIG. 1, a fluid injector includes a multi-part barrel 1 supporting at its forward end by means of a capnut 2 an atomiser assembly 3. The assembly is clamped within the capnut by means of a cylindrical body 4 which is secured at its rear end to a multi-part central tube 5. A tip sealing valve member 6 is disposed within the body 4 and comprises an elongate tip 7 formed on the end of a hollow stem 8 or securely attached to the end of the stem 8. The stem 8 is attached to a hollow piston 9 which is a close sliding fit in the body 4. The stem 8 is similarly a sliding fit in a reduced section portion of the body 4 so that the tip valve member 7 is capable of sliding axially in the body 4 and is biased forwardly by a spring 10. When no fluid is supplied to the injector the bias of the spring 10 is sufficient to urge the tip valve member 7 into a discharge passage 11 of the atomiser assembly 3.

A spill valve body 12 is connected to the rear end of the barrel 1 and a fluid connection 14 is made in the body 12. A further fluid connection 13 is made to the barrel 1 forwardly of the junction between the barrel and the spill valve body 12. The rear end of the central tube 5 is also connected to the spill valve body 12 and housed within the body 12 is an axially slideable spill valve member 15 consisting of a hollow piston 16, a valve seat 17 and a valve spindle 18 terminating in a piston 19. An adjusting screw 20 is housed in the spill body 12 coaxial with the valve member 15.

Fluid is supplied to one or other of the ports 13, 14 of the injector by way of a changeover valve which is separate from the injector and which is connected to suitable fluid delivery and return circuits. The changeover valve may be of the type illustrated in FIG. 2 and to be described in detail hereinafter.

With the components of the injector in their positions shown in FIG. 1 and with fluid supplied to port 14 on the injector and with the port 13 connected to a return circuit by way of the changeover valve, the paths taken by the fluid in the injector will now be described.

The path of the fluid, referred to as the first path, from the first port 14 is via a spill valve orifice 21,

through apertures 16' in th spill valve piston 16 and via the central tube to the tip valve member 6, passing via the hollow piston 9, the hollow stem 8, through an annulus 22 between the body 4 and valve tip 7, into an atomiser swirl passage 23. The return path, referred to as the second path, is via an annulus 24 formed between the barrel 1 and the central tube 5 to the second port 13. Although the discharge passage is closed off by the tip valve there is a continuous circulation of fluid through the injector and particularly in the vicinity of the discharge passage.

Due to velocity head losses there is a pressure drop across the annulus 22 and the atomiser swirl passages 23. Consequently the fluid pressure in the central tube 5 is higher than that in the annulus 24.

At the tip valve, fluid pressure is acting on the rear face of piston 9 and the forward face of the piston is subject tovthe lower fluid pressure in annulus 24 to which this face of the piston is connected via ports 26. This pressure is acting on an area defined by diameters D minus D which is consequently less than the rear face area of the piston 9 defined by diameter D Thus there is a force determined by the different pressures and the differences in area on which the pressures act which serves to maintain the tip valve 7 firmly in the discharge passage 11 to close off the discharge passage.

Ports 25 connect annulus 24 to the forward housing of the spill valve piston 16 and there is a pressure differential acting on the area defined by the diameter D, and D of the piston 16 which acts in a direction to maintain the spill valve seat 17 in the forward position shown in FIG. 1 with the piston abutting a stop member 5', and thus presenting the minimum restriction to the flow of fluid.

In order to condition the injector for discharge of fluid the changeover valve connects the fluid supply to the second port 13 on the injector and connects the first port 14 to a return circuit. The fluid supplied to the port 13 passes through the annulus 24 and by way of the port 26 on to the forward face of the tip valve piston 9 while a lower pressure, due to velocity head losses through swirl passages 23 and the annulus 22, acts on the rear face of the piston and this differential pressure causes the tip valve to move rearwardly to a positive stop 5A to open up the discharge passage 11.

The fluid applied to port 13 applies pressure by way of the port 25 on to the forward end of the spill valve piston 16 and the fluid pressure acts to displace the spill valve member to force the valve seat 17 towards the orifice 21 to close off the orifice or to force the piston 19 up into engagement with the adjustable screw 20. Thus the spill valve tends to close off the return path from the vicinity of the discharge passage to the port 14.

If the adjustable screw 20 is set so that the valve seat 17 closes off the orifice 21 then there will be no circulatory flow through the injector and no pressure losses to cause a differential pressure to act on the tip valve piston 9. Consequently the tip valve member would not move to open the discharge passage. To overcome this difficulty the piston 19 of the spill valve is made a good sliding fit in a hole 27 in the valve body 12 and when the injector is in the shut down, non-discharge condition, a space 28 between the adjustable screw 20 and the piston 19 fills with fluid.

When the fluid flow to the injector is reversed and the spill valve operates to close spill orifice 21, fluid in the space 28 acts as a dash pot serving to slow down the movement of the valve member 15 in order to allow sufficient time for the tip valve member 6 to operate to the discharge position. When this condition has been achieved the return fluid flow may be stopped by the seat 17 sealing the orifice 21 as there is a drop in fluid pressure due to the flow through the swirl passage 23 which is being discharged via the discharge passage 11. When the injector is in the discharge condition the return flow is entirely controlled by the piston of the spill valve relative to the orifice 21 together with the return fluid pressure set up by other external valves in the fluid return system.

The fluid flow to the injector when in the shut down, non-discharge condition, may be increased without increasing the flow rate through the injector, by means of an adjustment of the changeover valve, the ability to set the fluid flow at a particular rate when the injector is shut down is of importance in automatic oiler control since this permits the shutting down and starting up of burners without disturbing the total oil flow to the boiler.

A changeover valve suitable for use with the injector shown in FIG. 1 is shown in FIG. 2. The valve includes a body 29 having four ports 30, 31, 32 and 33 spaced apart along its length. Port 30 serves as an inlet port and port 31 serves as a return port. Ports 32 and 33 are connectable to ports 13 and 14 respectively on the injector. The adjacent ports are separated by internal walls each having an orifice 34, 35, and 36 respectively. An axially displaceable valve stem 37 is mounted within the body and extends through the orifices. Valve seats 38, 39 and 43 project outwardly from the valve stem. An end portion of the valve stem is drilled to a position beyond the valve seats 38 and 39 and ports 40 connect a bore 41 of the stem with an annulus 42 formed between the stem and the valve body.

The hollow end of the stem carries the valve seat 43 which is displaceable axially of the stem up to a stop 44. The valve seat is held against the stop by means of a spring 45 retained on the stem by a clip 46. A valve cap 47 carried at one end of the valve body has a spigot 48 which is coaxial with the passage in the valve stem. An O-ring seal 54 on the spigot serves to seal the end of the passage in the valve stem when the stem is moved over the adjacent end of the spigot. The opposite end of the valve stem carries a piston 49 with a suitable peripheral seal 50 which slides in a cylinder 51. Suitable connecting passages allow compressed air to be admitted as required to one side or other of the piston. An end cap 52 is attached to the cylinder 51 and an adjustable screw 53 carried by the cap locates coaxially with the valve stem 37.

With the changeover valve in the position shown in FIG. 2 fluid enters the valve body via the inlet port 30 and flows through the ports 40 into the passage in the hollow valve stem and out of the free end of the passage and out of the valve through the port 33. The valve members 38 and 36 seal off the orifices between the first and second ports and between the third and fourth ports respectively. The orifice between the second and third ports is open and fluid returns from the injector by way of the port 32 and discharges through port 31. The pressure of the fluid acting on the area defined by diameter D,-, serves to maintain the valve seat 38 firmly against the orifice 34 and the fluid supply pressure acting on the annulus defined between diameters D and D maintains valve seat 43 firmly in the orifice 36. The thrust of the spring also acts to hold the valve seat 43 on orifice 36 but the reaction of the spring on valve stem 37 decreases the pressure thrust of the seat 38 against the orifice 34.

However by designing the area defined by D the annulus area defined by D and D and the rate of its spring and its compressionn, the seating forces can be made equal. All other pressure forces are equalised by the geometry of the valve. In addition to the fuel pressure forces there is the thrust due to the air pressure acting on the piston 49. The unbalance of forces due to fuel pressure ensure that on loss of operating air pressure the changeover valve moves to the position shown in the figure consequently causing the injector to shut down and the system is thus of the fail-safe category. Adjustment of screw 53 causes the seat 38 to move away from the orifice 30 and allows fluid to flow from the inlet port 30 directly to the outlet port 31 thus bypassing the fuel injector. As the same fluid pressures are acting in the supply and return pipes to the injector the flow is unaltered through the injector for cooling purposes.

When the changeover valve is operated by applying compressed air to the outer face of the piston 49 the valve stem moves until the valve seat 39 closes off the orifice 35. Simultaneously spigot 48 enters the hole 41 and the valve seat 43 moves away from the orifice 36. The fluid flow now enters by the inlet port 30 and discharges by the port 32 and the returned fluid from the injector enters the valve through port 33 and discharges from the changeover valve via port 31. Simply by changing over the changeover valve the fuel injector is conditioned for fuel discharge.

FIG. 3 illustrates an embodiment of the invention in which provision is made for controlling and adjusting the circulation of fluid through the injector when the discharge passage is closed. With this arrangement it is not necessary for the changeover valve to have provision for bypassing the injector and consequently any form of four port changeover valve can be employed. In FIG. 3 a spill valve body is connected to the rear end of the barrel 1 and a fluid connection 64 is made in the body 60. The rear end of the central tube 5 is also connected to the spill valve body and housed within the body is an axially slideable spill valve member 65 consisting of a hollow piston 66, a tapered valve seat 67 and a valve spindle 68 terminating in a piston 69. An adjusting screw is housed in one end of the spill body 60 coaxial with the valve member 65.

The piston 69 and the forward end of the adjusting screw 70 are displaceable in the bore of a sleeve 71 displacable axially in the body 70 by an adjusting screw 72. The sleeve has an annular cut away portion 73 adjacent one end and the cut away portion connects with the port 64 in all positions of the sleeve. The cut away portion is in communication with the bore of the sleeve through ports 74 and an orifice 75. The forward end 76 of the sleeve constitutes a valve orifice which can be closed off by the valve seat 67 on the spill valve member.

This constructions permits the flow of fluid circulating through the injector when the tip valve is closed off to be adjusted independently of the flow of fluid through the injector when the tip valve is open. When the tip valve is closed and fluid is applied to the port 64, whatever the axial position of the sleeve 71, sufficient fluid enters through either orifice 75 or port 74 to apply a force to the tip valve to keep it closed and to the spill valve to keep it in abutment with the rear end of the tube 5. By displacing the sleeve to the left from the position shown in FIG. 3 the ports 74 are gradually opened up increasing the flow of circulating fluid to a maximum when the ports 74 are completely open.

When the fluid is supplied to the port 13 on actuation of the changeover valve, the tip valve is automaticallly opened and spill valve 68 is displaced to the right from the position shown in FIG. 3 until the piston 69 abuts the end of the screw machine 70. The valve seat 67 moves towards the valve orifice 76 to reduce or shut off the supply of fluid to the port 64. In this way the quantity of fluid discharged from the discharge passage of the injector through the tip valve is controlled.

The embodiment of the invention illustrated in FIG. 4 permits two different rates of flow to be obtainable when the tip valve is open. A spill valve body 80 is connected to the rear end of the barrel 1 and a liquid connection 81 is formed in the body. The rear end of the central tube 5 is connected to thebody and housed within the body is an axially slideable spill valve member 82 consisting of a hollow piston 83, a tapered valve seat 84 and a valve spindle 85 which is terminated in a piston 86.

The spindle 85 and the piston 86 are displaceable axially in a bore 87 of a cylindrical sleeve 88. The sleeve carries a piston 89 displaceable in. an enlarged bore 90 of the valve body 80 and an inlet port 9] enables a fluid under pressure to be supplied to the bore 90 on one side of the piston. The sleeve also carries an elongate stem 92 which is located in a hollow spigot 80A on the body 80. An end portion 93 of the stem is threaded and carries a pair of lock nuts 94. A capnut 95 encloses the stem 93 and the locknuts 94 and is screw threaded on to the periphery of the spigot 80A.

In the position shown in FIG. 4 the fluid injector has fluid supplied to the port 13 and some of the fluid is discharged through the discharge passage but much of the fluid is not discharged and returns along the tube 5, through the hollow piston 83, between the valve seat 84 and the valve orifice defined by the adjacent end of the sleeve 87 and out of the port 81. The pressure of the fluid in the injector acts on the piston 83 and causes it to be positioned to the position shown in FIG. 4 where it abuts against a stop 96. The fluid pressure in the injector also acts on the sleeve 88 to displace it rearwardly until either a shoulder 97 a buts against a wall 98 of the body 80 or the end of the stem 93 abuts the capnut 95. The position of the sleeve shown in FIG. 4 is an extreme position with the shoulder 97 abuting against the wall 98 but the sleeve may be displaced relative to the valve body to the left from the position shown by screwing the capnut 95 to displace the stem 92. The flow of fluid through the orifice and the end of the sleeve is thus adjusted to a particular value.

To increase the flow of fuel through the tip valve, the fuel returned through the spill valve to the port 81 is reduced by applying fluid under pressure to the port 91 to thereby displace the piston 89 and consequently the sleeve 87 until either the sleeve orifice is closed completely by the tapered valve seat or until the end face 99 of the spigot is abutted by the adjacent locknut 94 in which case a trickle of fluid is returned to the port 81. By adjusting the position of the locknuts 94 on the stem 93 the spill flow of fluid can be adjusted and hence the flow of fluid through the tip valve is adjusted.

In a further modification of the arrangement shown in H6. 3 the screw member 70 is reduced in length and a coil spring is fitted into the cavity between the end of the screw and the piston 69 with opposite ends of the spring abutting the screw and the piston respectively. The spring rate is low and the initial compression high so that for small movements the force required would be nearly constant. The flow control valve then becomes a constant differential control valve, maintaining a constant differential between the supply pressure and the return or spill line pressure and this in turn gives a wide turn-down of the firing flow as the supply pressure is varied.

I claim:

1. A fluid injector comprising a fluid discharge passage; a tip sealing valve for controlling fluid discharge through said passage; first and second ports for connection to a fluid delivery and a return circuit external of the injector; a first fluid path from said first port to the vicinity of said passage, a second fluid path from said vicinity of the passage to said second port, and a spill valve in said first fluid path; said spill valve and said sealing valve being arranged such that with fluid delivered to said first port only said spill valve is subjected to a fluid differential pressure to open said spill valve to permit the fluid to be delivered along said first path to said vicinity of the passage; and said sealing valve is subjected to a fluid differential pressure to close said passage; whereby the fluid returns via said second path to said second port, and with the fluid delivered to said second port only said tip valve is subjected to a fluid differential pressure to open said passage; and said spill valve is subjected to a fluid differential pressure tending to close said first path to thereby promote discharge through said passage; and means for adjusting said spill valve to permit a proportion of the fluid supplied to said second port to return along said first path to said return circuit; thereby controlling the proportion of the fluid supplied to said second port which is discharged through said passage.

2. The fluid injector as defined in claim 1, wherein said sealing valve includes an elongate tip displaceable into and out of sealing relation with said discharge passage, said tip being carried by a hollow piston displaceable in a cylinder; said piston having a pair of opposed faces of different areas, the face of greater area being in communication with said first fluid path, and the other face being in communication with said second fluid path.

3. The fluid injector as defined in claim 2, further comprising spring means for biasing said piston to a position in which said tip seals off said discharge passage.

4. The fluid injector as defined in claim 1, further comprising a tapered valve seat on said spill valve, associated with a valve orifice, said spill valve being displaceable by the differential fluid pressure applied to it with respect to said orifice.

5. The fluid injector as defined in claim 4, wherein said valve orifice is provided by a sleeve surrounding a portion of said spill valve, said sleeve being displaceable axially with respect to said valve seat between two extreme positions in which said valve orifice is substantially closed by said valve seat, and is not closed by the latter, respectively.

6. The fluid injector as defined in claim 5, wherein at least the extreme position of said sleeve, in which said valve orifice is not closed by said valve seat, is determined by an adjustable stop member.

7. The fluid injector as defined in claim 6, wherein said sleeve carries a piston displaceable in a cylinder in a direction to at least substantially close said valve orifice by a supply of fluid under pressure introduceable into said cylinder.

8. The fluid injector as defined in claim 4, wherein said spill valve includes a hollow piston displaceable in a cylinder; said piston having a pair of opposed faces of different surface areas, the face with the greater surface area being in communication with said first fluid path, and the other face being in communication with said second fluid path.

9. The fluid injector as defined in claim 8, wherein said means for adjusting the spill valve includes a screw the length of which is adjustable in the direction of displacement of said spill valve, said screw being engageable by said spill valve to limit the displacement thereof.

10. The fluid injector as defined in claim 9, further comprising a fluid-filled dash pot positioned between the engageable surfaces of said screw and said spill valve to provide a resistance to the displacement of said spill valve into engagement with said screw.

11. The fluid injector as defined in claim 9, wherein said valve orifice is provided by a sleeve surrounding a portion of said spill valve, and further comprising screw means for displacing said sleeve axially relative to said spill valve.

12. The fluid injector as defined in claim 11, wherein said sleeve has at least one port through the side wall thereof, said one port being included in said first fluid path, and the area of overlap between said one port and said first port being determined by the axial position of said sleeve.

13. The fluid injector as defined in claim 11, further comprising a spring interposed between said screw means and said spill valve, said spring having a low rate and initial high compression. 

1. A fluid injector comprising a fluid discharge passage; a tip sealing valve for controlling fluid discharge through said passage; first and second ports for connection to a fluid delivery and a return circuit external of the injector; a first fluid path from said first port to the vicinity of said passage, a second fluid path from said vicinity of the passage to said second port, and a spill valve in said first fluid path; said spill valve and said sealing valve being Arranged such that with fluid delivered to said first port only said spill valve is subjected to a fluid differential pressure to open said spill valve to permit the fluid to be delivered along said first path to said vicinity of the passage; and said sealing valve is subjected to a fluid differential pressure to close said passage; whereby the fluid returns via said second path to said second port, and with the fluid delivered to said second port only said tip valve is subjected to a fluid differential pressure to open said passage; and said spill valve is subjected to a fluid differential pressure tending to close said first path to thereby promote discharge through said passage; and means for adjusting said spill valve to permit a proportion of the fluid supplied to said second port to return along said first path to said return circuit; thereby controlling the proportion of the fluid supplied to said second port which is discharged through said passage.
 2. The fluid injector as defined in claim 1, wherein said sealing valve includes an elongate tip displaceable into and out of sealing relation with said discharge passage, said tip being carried by a hollow piston displaceable in a cylinder; said piston having a pair of opposed faces of different areas, the face of greater area being in communication with said first fluid path, and the other face being in communication with said second fluid path.
 3. The fluid injector as defined in claim 2, further comprising spring means for biasing said piston to a position in which said tip seals off said discharge passage.
 4. The fluid injector as defined in claim 1, further comprising a tapered valve seat on said spill valve, associated with a valve orifice, said spill valve being displaceable by the differential fluid pressure applied to it with respect to said orifice.
 5. The fluid injector as defined in claim 4, wherein said valve orifice is provided by a sleeve surrounding a portion of said spill valve, said sleeve being displaceable axially with respect to said valve seat between two extreme positions in which said valve orifice is substantially closed by said valve seat, and is not closed by the latter, respectively.
 6. The fluid injector as defined in claim 5, wherein at least the extreme position of said sleeve, in which said valve orifice is not closed by said valve seat, is determined by an adjustable stop member.
 7. The fluid injector as defined in claim 6, wherein said sleeve carries a piston displaceable in a cylinder in a direction to at least substantially close said valve orifice by a supply of fluid under pressure introduceable into said cylinder.
 8. The fluid injector as defined in claim 4, wherein said spill valve includes a hollow piston displaceable in a cylinder; said piston having a pair of opposed faces of different surface areas, the face with the greater surface area being in communication with said first fluid path, and the other face being in communication with said second fluid path.
 9. The fluid injector as defined in claim 8, wherein said means for adjusting the spill valve includes a screw the length of which is adjustable in the direction of displacement of said spill valve, said screw being engageable by said spill valve to limit the displacement thereof.
 10. The fluid injector as defined in claim 9, further comprising a fluid-filled dash pot positioned between the engageable surfaces of said screw and said spill valve to provide a resistance to the displacement of said spill valve into engagement with said screw.
 11. The fluid injector as defined in claim 9, wherein said valve orifice is provided by a sleeve surrounding a portion of said spill valve, and further comprising screw means for displacing said sleeve axially relative to said spill valve.
 12. The fluid injector as defined in claim 11, wherein said sleeve has at least one port through the side wall thereof, said one port being included in said first fluid path, and the area of overlap between said one port and saiD first port being determined by the axial position of said sleeve.
 13. The fluid injector as defined in claim 11, further comprising a spring interposed between said screw means and said spill valve, said spring having a low rate and initial high compression. 